A photolithographic tool is a device that exposes a reticle pattern onto a silicon or glass substrate, in which a reticle transfer system is a critical component responsible for transfer of a reticle between a reticle stage in an exposure unit and a reticle interface (reticle cassette).
The reticle transfer system functions essentially to transfer an unexposed reticle from the reticle cassette onto the reticle stage and the exposed reticle from the reticle stage back to the reticle cassette while ensuring that in these courses reticle marks in the reticle stage are maintained within a range that allows the marks to be captured by an alignment system.
In operation of the reticle transfer system, a reticle carrier is caused to move upward to a reticle cassette transfer position where the reticle cassette is opened to allow a robot to pick up a reticle therefrom. The robot then carries the reticle to a position at which it performs pre-alignment with the aid of a pre-alignment unit so that the reticle held by the robot is properly adjusted with respect to a reticle stage in terms of position. Thereafter, the reticle is placed on the reticle stage.
Conventionally, reticle supports arranged on a reticle stage are so spaced apart from one another that, during the pre-alignment of the reticle held by the robot, they are close to a pellicle frame attached to the reticle which is partially located within a space defined by them. If the reticle held by the robot is tilted at an excessive angle with respect to the reticle stage, it is likely for the pellicle frame to collide with the reticle supports. As a consequence, the pellicle frame may be damaged and the reticle may thus be contaminated and scrapped.